1. Field of the Invention
This invention relates to a power semiconductor device, and more particularly to a power semiconductor device having a superjunction structure.
2. Background Art
The ON resistance of a vertical power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) greatly depends on the electric resistance of its conduction layer (drift layer). The dopant concentration that determines the electric resistance of the drift layer cannot exceed a maximum limit, which depends on the breakdown voltage required for a pn junction formed by the base and the drift layer. Thus there is a tradeoff between the device breakdown voltage and the ON resistance. Improving this tradeoff is important for devices with low power consumption. This tradeoff has a limit determined by the device material. Overcoming this limit is the way to realizing devices with low ON resistance beyond existing power devices.
As an example MOSFET for solving this problem, a structure is known as a “superjunction structure”, which is formed by p-pillar layers and n-pillar layers buried in the drift layer. In the superjunction structure, a non-doped layer is artificially produced by equalizing the amount of charge (amount of dopant) contained in the p-pillar layer and the n-pillar layer. Thus, with retaining high breakdown voltage, a current is allowed to flow through the highly doped n-pillar layer, thereby realizing low ON resistance beyond the material limit. For retaining high breakdown voltage, it is necessary to accurately control the amount of dopant in the n-pillar layer and the p-pillar layer.
In such a MOSFET with a superjunction structure formed in the drift layer, the design of the edge termination structure is also different from that of conventional power MOSFETs. Because the edge termination section as well as the device section needs to retain high breakdown voltage, the superjunction structure is usually formed also in the edge termination section. In this case, when the amount of dopant in the n-pillar layer is equal to that in the p-pillar layer, the breakdown voltage of the edge termination section decreases more significantly than that of the device section (cell section). Thus some structures have already been devised for increasing the breakdown voltage of the edge termination section to prevent the overall decrease of breakdown voltage. In one structure, the p/n-pillar concentration in the edge termination section is made lower than in the device section. In another structure, the arrangement period of pillar layers is narrowed (see JP 2001-298190A). On the other hand, in a different structure, for increasing the breakdown voltage of the edge termination section, the edge termination section is formed from a high-resistance layer without a superjunction structure (see JP 2000-277726A).
However, in any of these structures, the superjunction structure is discontinuous between the device section and the edge termination section. In this discontinuous portion, that is, in the outermost p-pillar layer or n-pillar layer of the superjunction structure of the device section, the dopant concentration must be decreased to about half that in the cell section. For realizing such dopant concentration of the pillar layer varied with position, the dose amount of ion implantation must be varied with position, or the opening width of the implantation mask must be varied. Varying the dose amount with position leads to decreased throughput such as implantation being divided into twice. On the other hand, varying the mask width can be easily realized by varying the lithography mask width. However, a conversion difference occurs between the lithography mask and the resist mask used for actual implantation. Dispersion in this conversion difference is equivalent to dispersion in the amount of dopant. Thus, unfortunately, the edge termination structure, which is promising for high breakdown voltage in principle, is difficult to realize and susceptible to process dispersion.